XMRV is a newly discovered member of the gammaretrovirus genus of Retroviruses that has been recently associated with two human disorders, prostate cancer and chronic fatigue syndrome (CFS). Since it was first reported in 2006, XMRV has been intensely investigated, but conflicting data regarding XMRV detection have been reported, and no clear picture of prevalence, geographic distribution, disease association, or role in pathogenesis has emerged. The identification of XMRV in otherwise healthy individuals raises additional concerns regarding the safety of the blood supply. Several serious limitations prevent in-depth understanding of the role of XMRV in human infection and disease, including systems to detect and sequence XMRV nucleic acid. Currently, there is a lack of adequate XMRV nucleic acid detection systems with robust single-copy detection characteristics. In addition, understanding the origin, spread, and pathogenesis of XMRV is hampered by inability to obtain extensive phylogenetic information using standard population-based amplification and sequencing. We have initiated studies to detect and quantify XMRV by using real-time PCR techniques and have developed single-copy assays for analysis of XMRV RNA in plasma and whole blood and for analysis of XMRV DNA in PBMC and tissue-derived material. As a result of the expertise in our Virology Core, we are one of four laboratories in the United States testing panels of XMRV standards and samples. In the first analyses studying a panel of spiked blood samples, the Core assays demonstrated excellent detection characteristics with single-copy sensitivity. The development of XMRV real-time PCR assays incorporated an important design feature that permits distinguishing XMRV from typical endogenous retroviruses. This novel aspect in XMRV detection design affords particular advantages in XMRV detection in humans, as there are new data suggesting that other endogenous viruses may be present in humans as well as the described XMRV. As a consequence of this novel aspect of the assay, we are nowpreparing an Employee Invention Report for possible patent application. In the course of developing these quantification assays, it became clear that control studies are essential to rule out the possibility of contaminating substances generating artifactual signal. In addition to typical adventitious amplification, XMRV detection is particularly sensitive to murine contamination;the mouse genome contains hundreds if not thousands of copies of endogenous retroviruses, representing a substantial load of retrovirus that may be amplified in XMRV assays. To identify potential sources of contamination, we have developed sensitive assays to detect low-level mouse contamination, including single-copy assays for mouse genomic and mitochondrial DNA. In addition, we have also developed a quantitative assay for DNA of intracisternal A type particle (IAP), a virus amplified to a high degree in mouse cells. Taken together, this suite of amplification assays represents a comprehensive approach to detect XMRV in human-derived samples. The Core has also established new single-genome sequencing (SGS) approaches to analyze genetic diversity in the XMRV population. As we have demonstrated previously with HIV, SGS permits detailed analysis of viral population genetics. As XMRV may have substantial geographic distribution, SGS will be particularly useful in analysis of XMRV epidemiology. SGS will also provide a substantial measure of quality control and determine whether identified sequences are the result of laboratory contamination. These new assays will be essential in determining the role of XMRV in disease pathogenesis. We have established useful collaborations with Drs. W. Marston Linehan and Peter Pinto (Urologic Oncology Branch, CCR) to study samples from patients with prostate cancer, and with Drs. Frank Ruscetti and Kathryn Jones (Laboratory of Experimental Immunology, CCR) and Dr. Judy Mikovits (Whittemore Peterson Institute) to study patients with severe CFS. As immune deficiency may contribute to infection by XMRV, we have also established collaborations with NIAID to obtain samples from study individuals with immune deficiency, including HIV-infected patients and patients with both HIV infection and prostate cancer. In addition, we are collaborating with Dr. Vinay Pathak (HIV Drug Resistance Program, CCR) in a study of XMRV pathogenesis and prostate cancer as part of a Bench to Bedside Award to Dr. Pathak. Reports of XMRV infection in individuals with chronic fatigue and in otherwise healthy individuals raised concerns regarding new health risks. Within a year of these reports, we have optimized a series of detection and analytical assays with excellent performance characteristics. In the next year, we will apply these approaches to shed new light on the potential role of mouse-related viruses in human disease.